"Richard Mourdock is the conservative choice for Indiana," Palin, the 2008 GOP vice-presidential nominee, wrote on Facebook. "Senator Lugar’s 36 years of service as a Senator are appreciated, but it’s time for the torch to pass to conservative leadership in Washington that promises to rein in government spending now."

The Lugar campaign responded to the announcement by resending 2008 GOP presidential nominee Sen. John McCain's (R-Ariz.) endorsement and a new radio ad with McCain.

Palin's endorsements were a regular feature of the 2010 race, with the former Alaska governor making 64 endorsements. These endorsed candidates won 33 races and lost 21 -- of course, Palin's individual endorsement didn't always mean the candidate won the race. Also, unlike 2010, Palin has shut the door on a presidential run for 2012 and has no obvious path for higher office.

Mourdock led Lugar by five points in a poll released on behalf of Citizens United, which is backing Mourdock in the May 8 primary. He has been endorsed by other conservative groups such as the Club For Growth, The National Rifle Association and FreedomWorks.